Due to global warming concerns, there has been an increased interest in technology for capturing and sequestering CO2 from processed fluids. This can involve, for example, separating CO2 from other molecules present in a processed fluid (i.e., “capturing” it) and then compressing it, transporting it, and disposing of it, for example, by burying it in the ground (i.e., “sequestering” it). Coal-fired power plants account for more than half of the ˜3.8 billion tons of CO2 emitted from stationary sources in the United States, and the relatively dilute concentration of CO2 in their flue gas makes capture challenging. For example, CO2 may comprise only 4-14% of the total flue gas being emitted from the power plant, but the energy required to capture it can comprise around 60% of the total cost of the entire capture and sequestration process. Typical polymer films used as membranes today can separate gaseous species only based on differences in their diffusivity and solubility. Since the molecular diameters of CO2 and N2 are very similar, separation of CO2 by a polymer film is challenging, with N2 being much more abundant and creating a higher driving force for permeation through the membrane.
Technologies have been developed to capture CO2 using chemical scrubbing with chemicals having a selective and strong affinity for CO2 molecules. Currently, the best available capture system employs amine-based chemical scrubbing. The technology is effective in removing CO2; however, the cost is prohibitively expensive, estimated to be over $100 billion/year for U.S. Department of Energy's goal of 90% capture, and adding ˜85% to the cost of electricity. The fundamental reason lies in the chemistry associated with the process, in which CO2 is captured by reaction with 30% wt. aqueous amines. First, the kinetics of this process are inherently slow since the rate limiting step is transport through the liquid phase. Slow kinetics results in a need for large equipment to meet capture demands such as 90%, for example. Hence the capital costs associated with CO2 capture for a 550 megawatt (MW) coal-fired power plant are estimated to be ˜$740 MM. Second, the reaction products are relatively stable carbamate compounds which require large amounts of heat to release the CO2 and regenerate the amines; incidentally, most of this energy is wasted since 70% of the mass to be heated is water. As a result, the efficiency of the same 550 MW plant would drop from 36.8% to 24.9%.
In contrast to amine-based chemistry, complex aerobic organisms (including mammals) react CO2 with water toward the formation of bicarbonate ions, which are much more soluble in blood than CO2 itself. The unaided reaction of CO2 with water is slow, but in our body it happens almost instantaneously due to the action of carbonic anhydrase, one of the fastest enzymes known. Consequently, some groups have been investigating incorporating carbonic anhydrase in a membrane for CO2 capture. The enzyme is essentially dissolved in a liquid phase inside the pores of a polymer film as a “contained liquid membrane”. However, utilizing an enzyme outside the context of an organism (which can continuously regenerate and protect its enzymes with antioxidants) is a big challenge in terms of enzyme stability, availability and replenishment. The harsh environment of flue gases will be likely to chemically attack the embedded carbonic anhydrase enzyme. Such an approach will require tight control of temperature and contaminants, as well as provisions to replace denatured (degraded) enzyme at regular intervals. Since carbonic anhydrase is too complex to synthesize, its extraction from erythrocytes has significant costs, even if economies of scale are achieved.
Furthermore, carbonic anhydrase requires an aqueous environment to operate. This has two considerable drawbacks. First, like the liquid amines, the rate limiting step is transport through the liquid phase, which will set the ceiling for the membrane's performance regardless of how fast the enzyme converts CO2 to bicarbonate ions. Second, as the liquid phase is kept at a minimum volume to improve kinetics, even extremely low levels of sulfur dioxide from the flue gas will accumulate over time and turn to sulfuric acid, quickly decreasing the pH below the narrow range for optimal carbonic anhydrase function in the membrane. As the enzyme progressively loses its catalytic activity, the efficiency of the process continues to decrease until the membrane is rendered non-functional. As a consequence, the membrane will need to be replaced frequently, leading to high operation costs.